Fourier spectroscopy for analyzing the composition of a sample typically employs a two-beam interferometer such as a Michelson interferometer wherein a light beam is divided by partial reflection into two separate wavefronts, one directed along a fixed-length arm and the other directed along a variable-length arm which is varied to cause intensity modulation. In the usual rapid-scan mode of operation, a collimated light beam in the spectrometer is modulated by scanning one of the interferometer mirrors to produce a constant modulation rate. The resulting modulation signal, or interferogram, as modified by interaction with a sample under observation, is provided to a detector for detection and then Fourier transform (FT) processing.
In the typical Michelson interferometer, the scanning mirror is displaced in the direction of the light beam by a suitable mechanical drive mechanism causing the intensity of the central spot in the interference pattern to fluctuate as a function of the position of the movable mirror. The movable mirror typically undergoes translational displacement in a reciprocating manner. This high speed, reciprocating movement requires the use of a high precision linear bearing which substantially increases the cost of the interferometer. In addition, the requirement to displace the interferometer's scanning mirror at a constant velocity requires the use of a servo control loop which also contributes to the complexity and expense of the interferometer. The scanning mirror must be brought up to speed and be under the control of the servo loop during data collection. At the end of the scan, the mirror decelerates, stops, accelerates in the opposite direction, again comes under the control of the servo loop for the taking of data, again decelerates and stops. The next cycle begins with the acceleration of the scanning mirror in the opposite direction, with this sequence repeated. This type of reciprocating motion also requires considerable energy and gives rise to large momentum transfers to other instrument components frequently resulting in optical instabilities. Finally, this linear translation approach is generally limited to a maximum scanning mirror velocity of only 3 cm/sec and is characterized by a corresponding limited modulation range.
This type of interferometer is also highly sensitive to angular tilt and lateral shear (horizontal and vertical translation) of the scanning mirror which also reduces the modulation of the interferogram. One approach to reducing the effect of mirror tilt employs a retroreflector in the form of a cube corner reflector which directs the reflected beam along the same path as the incident beam. One attempt to avoid the problems encountered with translational, reciprocating displacement of the scanning mirror is disclosed in U.S. Pat. No. 4,383,762. This patent discloses a two-beam interferometer for Fourier spectroscopy including a rigid pendulum structure attached to a movable retroreflector. The swing of the rotatably journaled pendulum confines the retroreflector to movement in a single plane during scanning. This approach has a limited duty cycle and also requires a reciprocating, translational drive arrangement including magnets or springs, or a combination of both.
The present invention addresses the aforementioned limitations of the prior art by providing a Michelson interferometer with an orbiting retroreflector as the scanning element for essentially eliminating the sensitivity to the scanning element angular tilt and lateral shear, while providing a wide modulation bandwidth.